A. Field of the Invention
The present invention is directed to an improved method and composition for the surface treatment of a copper foil. More particularly, the present invention is directed to a method and composition for reducing an oxide whisker coating on the surface of a copper foil such that the reduced copper surface retains sufficient whisker structure to provide improved adhesion to a resinous surface bonded thereto. The present invention is useful because it is capable of producing single-sided, double-sided, and multi-layer circuit boards that exhibit good thermal and mechanical strength and good acid resistance.
B. Background of the Invention
Copper foil and/or film (hereinafter "copper film") is used in the preparation of single-sided, double-sided, and multi-layer circuit boards. In the preparation of those circuit boards, at least one surface of the copper film is bonded or laminated to a resinous substrate such as an epoxy, a polyamide, or a phenolic which may or may not be reinforced with glass or other fibers. Ideally, the resin should bind to the copper with sufficient strength to prevent separation during subsequent procedures, such as soldering or during the thermal and mechanical stresses imposed by actual use.
However, in reality, it is difficult to achieve good bond strengths between the ordinary smooth surface of a copper film and the corresponding surface of a resin. It was theoretically known that increasing the surface area of the copper film, such as by roughening its surface, is capable of producing greater bond strengths. Several approaches to increasing the surface area of the copper, and thus, the potential bonding strengths to resin, have been disclosed in the prior art. U.S. Pat. No. 2,364,993 first taught the oxidation of a metallic copper surface to a black oxide layer using an alkaline chlorite solution at 200.degree. F. for five minutes. Subsequently U.S. Pat. Nos. 3,177,103, 3,198,672, 3,240,662, 3,374,129 and 3,481,777 applied this technology to resin-copper bonding in printed circuit boards.
Other methods for providing a copper oxide layer on a copper film include oxidation of the copper surface with hydrogen peroxide, such as disclosed in U.S. Pat. No. 3,434,889, or with alkaline permanganate, such as disclosed in U.S. Pat. No. 3,544,389. The resultant copper oxide layers are layers of copper II (cupric) oxide and are referred to in the an as a "brown oxide" or a "black oxide" layer (collectively herein "a black oxide"). Ideally, the cupric oxide layers are composed of long needles or whiskers (hereinafter "whiskers") of cupric oxide.
A problem with many of the copper oxide layers was that they were too thick and fuzzy to provide good mechanical strength. Another problem was the mixed formation of cuprous oxide near the surface and cupric oxide away from the surface. Subsequent patents, such as U.S. Pat. No. 4,409,037 (Landau) which issued on Oct. 11, 1983, optimized reagent concentrations to produce a brown colored oxide layer that allegedly exhibited peel strengths greater than about 5 pounds per inch. Other methods for producing black oxide layers are disclosed in Rider et al., "Printed and Integrated Circuitry", New York, N.Y., McGraw-Hill 1963 at pp. 24-27.
Although the copper oxide layers greatly increased the surface area of the copper, they still suffered from two additional problems. The first problem, as described in U.S. Pat. No. 4,997,516 (Adler), which issued on Mar. 5, 1991, is that the "[c]opper oxides such as cupric/cuprous oxides are soluble in acid, and serious difficulties may arise as a result of later process steps which involve the use of acid." ['516 patent at col. 3, lns. 5-8.] For example, sulfuric and hydrofluoric acids are frequently used to remove smears and glass fiber fragments respectively in the through holes of circuit boards. Each acid is capable of attacking and dissolving the black copper oxide whiskers at the interface of the resin and copper film to form what is known in the art as a "pink ring". The "pink ring" forms due to the pink color of the copper surface as it appears through the resin after the black (or brown) oxide copper has been dissolved away. Such "pink rings" may extend several millimeters inward away from the hole. In the art, an increasing number and diameter of "pink rings" is associated with decreasing quality and bond strength in a circuit board.
Additional problems associated with using a copper oxide layer include the fact that the copper oxide layer is "brittle", has "poor mechanical properties" and a tendency "to get wet and contaminated easily." [See U.S. Pat. No. 5,506,200 (Chen), which issued on Apr. 9, 1991 at col. 1, lns. 28-30.] Chen's approach to solving these problems was to reduce the copper oxide layer "to cuprous oxide but not metallic copper" ['200 at col. 3, lns. 5-7] at a neutral to alkaline pH (i.e., pH "7-12") [ e.g., '200 at col. 3, ln. 15]. Although Chen's process reduced the radius both of laminate voids from 3-5 mils to less than 1 mil and of "pink rings" from 11-14 mils to 4-6 mils, Chen's process did not completely solve the problem. Accordingly, it is an object of the present invention to provide a surface on a copper film that is suitable for bonding to a resin and that has minimal amounts of the acid susceptible cuprous or cupric oxides thereon.
Adler taught that reducing the copper oxide whiskers to metallic copper with a reducing agent, such as formaldehyde, produced "a mass of copper particles or powder rather than a microstructure of discrete copper whiskers." ['516 at col. 5, lns. 15-16.] According to Adler, the powdery copper surface is objectionable for the following reasons:
The copper powder: (a) significantly reduces peel strength,
apparently through loss of the desired reticulated
microstructure; (b) is conductive and may cause "shorting"
between conductor lines in the printed circuit made with
that copper foil treatment; (c) will be encapsulated in the
resinous substrate during lamination; and (d) is difficult or
impossible to selectively remove.
['516 (Adler) at col. 5, lns. 49-57.] By way of example, Adler reduced a black oxide surface, having an original peel strength of 6.0 to 6.5 lb/inch, with formaldehyde. The resulting reduced surface had a peel strength of 3 to 4.5 lb/inch. Adler similarly teaches that when a black copper oxide layer was reduced to cuprous oxide with hydrazine and the resulting cuprous oxide layer was dissolved with HCl-NaCl, the surface was powdery and only exhibited a peel strength of 2-3 lb/inch. Likewise, Adler teaches that alkaline sodium hydrosulfite (Na.sub.2 S.sub.2 O.sub.4) reduced a black copper oxide surface to "a powdery copper product" similar to that produced by formaldehyde. However, Adler states that if "gelatin" is added to the alkaline sodium hydrosulfite, the black copper oxide first turns to a "greenish material," which comprises "hydrous cuprous oxide," before becoming fully reduced to powdery copper. ['516 at col. 6, lns. 28-32.] According to Adler, if the reduction process is "interrupted" while the surface is still green, the cuprous hydroxide can be selectively dissolved in a subsequent step with HCl-NaCl to leave behind a non-powdery, copper whisker surface. [See '516 at col. 6, lns. 32, and 40-41.] Adler reports that the resulting non-powdery copper whisker surface had a peel strength of 5.9 lb/inch after lamination. However, this peel strength is less than the "6.0 to 6.5 lb/inch" peel strength reported by Adler for the original black oxide surface.
Accordingly, it is an object of the present invention to provide a method for bonding a copper film to a resin that provides good mechanical and thermal strength but that substantially eliminates the "pink ring" phenomenon and that eliminates the brittleness and the poor mechanical properties associated with bonding to a copper oxide layer.
It is another object of the present invention to provide a method that produces a surface on a copper film that has a good peel strength at least as good as the conventional black oxide surface.
It is a further object of the present invention that the method not require the presence of protein (gelatin) or polymeric moderating agents.